Light emitting diodes (LEDs) are attractive candidates for replacing conventional light sources such as incandescent lamps and fluorescent light sources. LEDs have substantially higher light conversion efficiencies than incandescent lamps and longer lifetimes than both types of conventional light sources. In addition, some types of LEDs now have higher conversion efficiencies than fluorescent light sources and still higher conversion efficiencies have been demonstrated in the laboratory. Finally, LEDs require lower voltages than fluorescent lamps, and hence, are better suited for applications in which the light source must be powered from a low-voltage source such as a battery or an internal computer DC power source.
Unfortunately, LEDs produce light in a relatively narrow spectral band. To replace conventional lighting systems, LED-based sources that generate light that appears to be “white” to a human observer are required. A light source that appears to be white and that has a conversion efficiency comparable to that of fluorescent light sources can be constructed from a blue LED that is covered with a layer of phosphor that converts a portion of the blue light to yellow light. Such light sources will be referred to as “phosphor converted” light sources in the following discussion. If the ratio of blue to yellow light is chosen correctly, the resultant light source appears white to a human observer. To provide the correct ratio, the thickness of the phosphor layer must be controlled. In addition, the uniformity of the phosphor layer over the die on which the LED is fabricated must be maintained to prevent variations in the color of the light over the light-emitting surface of the light source.
Cost, measured in terms of lumens of light produced per dollar, is an important concern in any light source that is directed to replacing conventional light sources. The cost of packaging the dies represents a significant fraction of the cost of the final light source. The packaging cost is increased by the need to capture light leaving the sides of the die. A significant fraction of the light produced in the blue LEDs used in phosphor converted light sources is trapped between the top and bottom surfaces of the die by internal reflection due to the difference in the index of refraction between the materials from which the dies are constructed and the surrounding medium. A significant fraction of this trapped light leaves the die through the sides of the die. To improve the light output of the die, a reflector is typically included in the light source to redirect the light leaving the side surfaces of the die such that the light leaves the die in the same direction as the light leaving the top surface of the die.
The packaging costs associated with providing a constant thickness of phosphor and a reflector are substantial. For example, in one design, the reflector is provided in the form of a cup that has reflective walls. The cup is mounted on a substrate that includes electrical traces for powering the die. A portion of the substrate is exposed through an opening in the bottom of the cup. The die is mounted on the portion of the substrate that is exposed through the bottom of the cup, and connected to the electrical traces. The cup is then filled with a suspension of phosphor particles in a material that can be cured to provide a solid layer of material in which the phosphor particles will remain suspended. Processes based on epoxy or silicone based materials for suspending the phosphor particles are known to the art.
These processes are difficult to automate in a manner that guarantees uniformity of the light emitted across the die. In particular, the fraction of the light that leaves the top surface of the die and is converted by the phosphor layer must be the same as the fraction of the light leaving the side surfaces of the die and is converted by the phosphor layer. In these processes, the phosphor layer is formed in the cup during the final assembly of the light sources on the light source assembly line by curing a carrier material that contains the phosphor particles. The phosphor particles tend to settle in the carrier material during the dispensing phase. To prevent the settling, the suspension must be augmented with various materials that slow the settling. In addition, the carrier must be formulated to provide a short curing time to prevent the particles from settling around the die in the cup. If the particles settle in the bottom of the cup, the amount of phosphor through which light leaving the sides of the die must pass is significantly different than the amount of phosphor through which the light leaving the top of the die must pass. As a result, the light that is redirected by the reflector has a different color than the light that leaves the top surface of the die. Hence, the light source exhibits a variation in color across the surface of the light source.
In addition, the process of providing individual cups for each die typically involves a number of additional fabrication steps. In the simplest designs, the cups are individually attached to the underlying substrate after the cups are fabricated. While designs in which the cups are created in a layer of material that is positioned over a number of dies have been suggested, the generation of the “cup layer” still represents a significant fraction of the cost of the final light source.